US 7466484 B2
The invention relates to an optical element including a substrate, a wire grid on one side of said substrate, and prismatic light collimating features on the other side of said substrate
1. An optical element comprising a substrate, a wire grid on one side of said substrate, and prismatic light collimating features on the other side of said substrate, wherein the wire grid comprises a plurality of parallel conductors having a pitch (P): a width (W), a height (H), and a length (L); a fill-factor (W/P) is in the range of approximately 0.18 to approximately 0.25; and an aspect ratio (L/P) that is larger than approximately 10, and wherein the conductors have an average length Lav and an average pitch Pav; and are arranged in a substantially random fashion parallel to each other and have an average aspect ratio of Lav/Pav is greater than aprroximately 10.
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is a width of the conductors as a function of height z.
15. A method of forming a wire grid comprising providing a substrate having dielectric wire features, coating said substrate and dielectric wire features with metal, removing the metal from the substrate and the portion of the dielectric wire features furthest from said substrate wherein the wire grid comprises a plurality of parallel conductors having a pitch (P), a width (W), a height (H), and a length (L); a fill-factor (W/P) is in the range of approximately 0.18 to approximately 0.25; and an aspect ratio (L/P) that is larger than approximately 10, wherein the wire aid comprises a plurality of parallel conductors having a pitch (P), a width (W), a height (H), and a leneth (L); a fill-factor (W/P) is in the range of approximately 0.18 to approximately 0.25: and an aspect ratio (L/P) that is lamer than approximately 10, and wherein the conductors have an average length Lav and an average pitch Pav; and are arranged in a substantially random fashion parallel to each other and have an average aspect ratio of Lav/Pav is greater than approximately 10.
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This is a continuation-in-part application of U.S. application Ser. No. 10/948,345 filed Sep. 23, 2004, entitled “LOW FILL FACTOR WIRE GRID POLARIZER AND METHOD OF USE” by Xiang-Dong Mi et al.
A wire grid polarizer in combination with other optical components is described for use in enhancing light efficiency. A process of making a low fill factor wire grid polarizer is described.
Light-valves are implemented in a wide variety of display technologies. For example, display and microdisplay panels are gaining in popularity in many applications such as televisions, computer monitors, point of sale displays, personal digital assistants and electronic cinema to mention a few applications.
Many light valves are based on liquid crystal (LC) technologies. In some LC technologies light is transmitted through an LC device (panel), while in others the light traverses the panel twice, after being reflected at a far surface of the panel.
In operation, an external field or voltage is used to selectively rotate the axes of the molecules of the LC material. As is well known, by application of this voltage across the LC panel, the direction of the LC molecules can be controlled and the state of polarization of the reflected light is selectively changed. As such, by selective switching of the transistors in the array, the LC medium can be used to modulate the light with image information.
In many LCD systems, light modulation results in dark-state light at certain picture elements (pixels) and bright-state light at others, where the polarization state governs the amount of light transmitted by each pixel element. Thereby, an image is created on a screen by the selective polarization transformation by the LC panel and optics to form the image or ‘picture.’
While LCD devices are becoming ubiquitous in display and microdisplay applications, there are certain drawbacks associated with known devices. For example, in known devices some of the light from the light source may be irrecoverably lost and the overall brightness of the image adversely impacted.
This loss of light in display systems may be from a variety of sources within the display system. In many LCD systems, the light from a source is selectively polarized in a particular orientation prior to being incident on the LC panel. This selective polarization may be effected by an absorptive polarizer. The LC panel may have a voltage selectively applied to orient the molecules of the material in a certain manner. The polarization of the light that is incident on the LC panel is selectively altered upon traversing through the LC layer. Light in one linear polarization state is transmitted by a polarizer (often referred to as an analyzer) as the bright state light; while light of an orthogonal polarization state is absorbed by the analyzer as the dark-state light. As can be appreciated, approximately one-half of the light energy is lost in such a system, due to absorption by the first linear polarizer.
In other display and microdisplay structures, a reflective polarizer may be used for polarization selectivity. Sometimes, the reflective polarizer is placed between a light source and the LC panel in order to provide light (polarization) recycling, wherein one polarization state is desirably transmitted by the reflective polarizer and one polarization state is reflected back toward the light source. In known systems using reflective polarizers, there are competing objectives that impede the polarization recycling. These competing objectives include light transmission efficiency and polarization extinction. For example, wire-grid polarizers may be used as the reflective polarizer. Known wire-grid polarizers have a relatively high extinction ratio. Such known wire-grid polarizers are not desirable in light recycling because the total light throughput is unacceptably low. This can result in unacceptable contrast and brightness in the viewed image.
Alternatively, plastic reflective polarizers comprising multilayer films or immiscible layered materials may be used in displays. Known plastic polarizers have a relatively low extinction ration and relatively high transmission. Such known plastic polarizers are not desirable in light recycling when used with very strong backlights as the plastics can ‘yellow’ due to light absorption. This can result in unacceptable coloration of the viewed image. An apparatus that overcomes at least the drawbacks of known devices described above is disclosed in U.S. application Ser. No. 10/948,345, filed Sep. 23, 2004. However, some problems remain, for example, additional separate prismatic light controlling film or components are needed, which increases the complexity of the entire display system.
The problem to be solved is to reduce the number of individual optical elements in a display system, and increase the display environment stability.
It is an object of the invention to provide an integrated optical element having a wire grid, which has a capability of recycling light of undesired polarization, and collimating light.
It is another object to provide an integrated optical element having a wire grid, which has reduced sensitivity to change in heat and humidity.
These and other objects of the invention are accomplished by an optical element comprising a substrate, a wire grid on one side of said substrate, and prismatic light collimating features on the other side of said substrate.
The invention provides enhanced optical efficiency, fewer number of optical elements, less sensitive to heat and humidity.
The invention is best understood from the following detailed description when read with the accompanying drawing figures. It is emphasized that the various features are not necessarily drawn to scale. In fact, the dimensions may be arbitrarily increased or decreased for clarity of discussion.
In the following detailed description, for purposes of explanation and not limitation, example embodiments disclosing specific details are set forth in order to provide a thorough understanding of the present invention. However, it will be apparent to one having ordinary skill in the art having had the benefit of the present disclosure, that the present invention may be practiced in other embodiments that depart from the specific details disclosed herein. Moreover, descriptions of well-known apparati and methods may be omitted so as to not obscure the description of the example embodiments. Such methods and apparati are clearly within the contemplation of the inventors in carrying out the example embodiments. Wherever possible, like numerals refer to like features throughout.
Briefly, and as described in detail in conjunction with example embodiments, a wire grid polarizer and a display system including a wire grid polarizer are disclosed. The wire grid polarizer includes a plurality of conductors having a width, a height and a pitch. In an example embodiment, the wire grid polarizer has a duty cycle chosen to substantially optimize the total light throughput, which is a function of the transmittance of light of a first polarization state through and the reflectance of light of a second polarization state from the wire grid polarizer.
In one example embodiment, the ratio of the height to the pitch of the wire grid polarizer is chosen to substantially optimize total light throughput. In this manner, a greater percentage of the light may be reflected and then transmitted through the wire grid polarizer and the absorptive polarizer, and thereby improve the brightness and contrast of an image provided through the display system.
In another example embodiment the reflected light may be depolarized using a diffusive reflector and reflected back to the polarizer. In yet another example embodiment, the reflected light may traverse a quarter wave retarder, be reflected by a reflector, and traverse the quarter wave retarder a second time before being incident on the wire grid polarizer.
In operation, unpolarized light from a light source(s) 104 is provided through a light guide 105, which may have a diffusive reflector 106 disposed over one or more outer surfaces. As is readily appreciated by one of ordinary skill in the art, the light guide 105, the diffusive reflector 106 and the light management film(s) are useful in providing a uniform light distribution to the light valve 101, with an angular distribution of light that is designed to meet the angular field of view required by an end-user. For example, a laptop computer with a brightness enhancement layer has a viewing angle that is typically on the order of approximately ±20 degrees off-center axis. Further details of backlight assemblies and the components thereof are described in detail in U.S. patent application Ser. No. 10/857,515 entitled “Diffuse Reflector Films for Enhanced Liquid Crystal Display Efficiency” to X. Mi et al., filed May 28, 2004 and assigned to the present assignee. The disclosure of this application is specifically incorporated herein by reference.
Light 107 from the light guide 105 is incident on the reflective polarizer 110, which transmits light 108 of a first polarization state (perpendicular to the plane of the page) and reflects light 109 of a second polarization state (parallel to the plane of the page). In the present example embodiment, this light is incident on the light guide 105, and via the diffusive reflection layer 106 is converted to unpolarized light and reintroduced with light 107 to the device 100. In this manner, the light 109 may be reintroduced, or recycled, thereby providing an improved optical efficiency and improved optical performance through improved contrast and brightness.
As can be readily appreciated, the greater the amount of light 108 that is transmitted, the greater the optical efficiency. In accordance with example embodiments, an improvement in the transmission of the light 108 is effected by increasing the transmittance of the reflective polarizer 110 and thus reducing the amount of light that is lost. As will become clearer as the present description continues, this is realized via an improved reflective polarizer of example embodiments.
In accordance with an example embodiment, the reflective polarizer 110 is a wire grid polarizer that has an improved transmittance and a reduced loss when compared to known reflective polarizers, such as other wire grid polarizers. An illustrative wire grid polarizer 200 is shown in
The wire grid polarizer 200 of the example embodiment of
In accordance with an example embodiment, the substrate 201 may be glass, polymer, or crystalline material that is transparent to the light at the chosen wavelength or wavelength range and provides good adhesion to the conductors 202. The substrate may or may not be diffusive to light. The conductors 202 are usefully aluminum or other suitable conductors such as copper, gold, and silver. Typically the more conductive the material chosen for the conductor 202 is in the targeted wavelength range, the better the wire grid polarizer's performance. For example, a good choice for the conductive material is aluminum, as it provides for a broadband reflective polarizer substantially covering the full visible spectrum.
Furthermore, the polarizer 200 may be fabricated using known fabrication techniques. These fabrication techniques include lithography and etching and electron beam etching techniques, to name only a few. It is noted that the fabrication technique chosen must afford the feature sizes, pitch and uniformity discussed below.
A variety of well-known techniques can be used to fabricate the reflective polarizer device, and the choice of fabrication techniques is not critical to the invention as long as the appropriate specifications for the surface texture are met. One method involves depositing and patterning a film of aluminum or silver directly onto a textured substrate. A variety of deposition and patterning techniques common to the semiconductor industry could be used to form a well-defined pattern over the textured surface so long as the method provides a sufficient depth of field. Holographic lithography, as described by Garvin in U.S. Pat. No. 4,049,944, or by Ferranti in U.S. Pat. No. 4,514,479 is a method that offers sufficient resolution and depth of field.
The desired surface texture of the underlying substrate 201 may be effected by mechanical abrasion of the substrate, such as grinding or sand-blasting or by chemical abrasion of the substrate, such as the well known use of dilute hydrofluoric acid to etch glass. Alternatively, a deformable material could be deposited on the substrate and then textured by embossing or by radiation assisted casting, as described by Shvartsman in U.S. Pat. No. 5,279,689. In still another method, the textured surface could be embossed or cast onto a continuous plastic film (for example, using the method described by Blenkhorn in U.S. Pat. No. 4,840,757) that may then be laminated to a rigid substrate.
Still another method for fabricating the diffuse reflective polarizer is to deposit and pattern the grid of parallel conductors on a smooth surface comprised of a polymer film over a rigid substrate, and then emboss the surface texture using the method described by Sager in U.S. Pat. No. 5,466,319. Other combinations of fabrication techniques are surely possible within the scope of the example embodiments. It is noted that the U.S. patents referenced in connection with the illustrative fabrication techniques are hereby incorporated herein by reference.
The wire grid polarizer 200 provides separation of orthogonally polarized states of light incident thereon. For example, light 206 having orthogonal polarization components in first and second polarization states 207 and 208, respectively, is incident at an angle of incidence (θ) (often referred to as the polar angle) with respect to the normal 209 to the plane of the polarizer 200. Illustratively, the first polarization state 207 is S-polarized light, which is perpendicular to the plane of the page; and the second polarization state is P-polarized light, which is parallel to the plane of the page.
Through the function of the polarizer 200, light 210 of the first polarization state (S-polarization) 207 is reflected from the polarizer 200 and light 211 of the second polarization state (P-polarization) 208 is transmitted through the polarizer. It is further noted that in certain example embodiments the viewing angle may be also measured from a normal to the display surface and thus the wire grid polarizer. It is noted that in some consumer applications, the viewing angle is on the order of approximately +20 degrees to approximately ±60 degrees. As such, it is useful to optimize the light output within this angular range.
In certain display applications, it is useful to optimize the amount of light that is incident on the light valve (e.g., light valve 101) and thus to the end-user. As referenced previously, this is effected in example embodiments by increasing the amount of light 211 that is transmitted and by minimizing the loss of light energy that occurs during the polarization separation process of the polarizer 200. In certain illustrative embodiments, this may be effected by providing a duty cycle or fill factor that lies within a certain range. Illustratively, this is effected by determining a value of the pitch 205, P, and varying the width 204, W, to realize the desired duty cycle, W/P. This term (W/P) is often referred to as the fill-factor.
As will be appreciated by one of ordinary skill in the art, the width, W, of the conductors 202 directly impacts the reflection and absorption of light energy of the incident light 206. To this end, the greater the width 204, the greater the amount of S-polarized light that is reflected and the greater the amount of light energy that is absorbed by the conductive material of the polarizer 200. However, the greater the width 204, the less the amount of P-polarized light that is transmitted. As such, and as will become clearer as the present description continues, to the extent possible, it is useful to choose a proper figure of merit by varying the ratio of the width to pitch of the conductor elements.
In other example embodiments, a comparatively improved total light throughput is realized by providing a ratio of the height 203, H, to the pitch 205, P, within a certain range. To wit, the greater the height 203, the greater the amount of S-polarized light that is reflected. However, the greater the height 203, the less the amount of P-polarized light that is transmitted. As such, it is also useful, to choose a proper figure of merit by varying the ratio of width to pitch.
By providing a certain fill factor, or by providing a certain ratio of the height 203 to the pitch 205 (H/P), or both, the wire grid polarizer provides an improved total light throughput and a reduced loss of light energy. It is noted that the improved light throughput of the example embodiments is effected with an extinction ratio (e.g., Tp/Ts where Tp and Ts are the transmittance of S-polarized and P-polarized light, respectively) that is significantly less than known wire grid polarizers. To this end, in known wire grid polarizers it is desired to provide an extinction ratio of 100 or 500 or even 1000. Extinction ratios of this magnitude are desired in many applications in order to improve the contrast ratio of the system. In order to achieve these relatively high extinction ratios, it is necessary to provide a wire grid polarizer that has conductors with dimensional widths and heights that can result in unacceptable reflection and absorption. As such, known wire grid polarizers are not efficient in applications where light recycling is desired.
In accordance with an example embodiment the extinction ratio is on the order of approximately 5:1 to approximately 20:1. These relatively low values notwithstanding, the contrast ratios realized using the wire grid polarizer of example embodiments in systems such as those of the example embodiments of
In example embodiments it is useful to substantially optimize the total light throughput that is a combination of the transmitted light Tp 211 of the second polarization state and the reflected light Rs of the first polarization state 210. This combination fosters an improved optical efficiency in systems such as those of the example embodiments of
In accordance with certain example embodiments, the wire grid polarizer 110, 200 is used as a polarization recycling device, and via a rather straight-forward analysis, the total light throughput, T1, that is transmitted through the wire grid polarizer and in the transmission axis of the rear polarizer 102 is given by:
Eqns. 1 and 2 apply to systems of the example embodiments where the recycling is effected using a diffusive reflector such as reflective layer 106. It is noted that R accounts for the reflectivity of the recycling reflective film, or the efficiency associated with each light recycling. In an ideal case, R is equal to 1, which means that there is no light loss in the light recycling. When R is less than 1, there is some light loss in the light recycling path. It is also noted that T1 is also referred to as the figure of merit for such diffusive reflector-based recycling systems.
In systems that incorporate a reflector and a quarter-wave retarder to effect desired light recycling, the figure of merit is given by the total light, T2, which is transmitted through the wire grid polarizer and in the transmission axis of the rear polarizer 102:
Again, R is the efficiency associated with each light recycling.
In keeping with the example embodiments, regardless of the type of recycling used, it is beneficial to optimize the figure of merit for the respective system in order to improve the amount of light (brightness) of the image formed via the displays of the example embodiments.
In general, six parameters (Ts, Tp, Rs, Rp, As, Ap) are required to describe the optical properties of a wire grid polarizer at a given angle of incidence, where Ts, Rs, As are the Transmittance, Reflectance, and Absorbance of the S-polarized light, respectively, and Tp, Rp, Ap are the Transmittance, Reflectance, and Absorbance of the P-polarization, respectively. They satisfy Ts+Rs+As=1 and Tp+Rp+Ap=1. Therefore, there are 4 independent parameters.
Equations 1 through 3 indicate that the two parameters Tp and Rs are particularly important for polarization recycling. Both Tp and Rs can be controlled by the duty cycle (W/P).
In general, a high duty cycle wire grid polarizer produces a high reflectance Rs and a low transmittance Tp, and a low duty cycle wire grid polarizer produces a low reflectance Rs and a high transmittance Tp. As suggested by U.S. Pat. No. 6,108,131 to Hansen et al., when the total efficiency is defined as the sum of Rs and Tp, known wire grid polarizers are likely to have a duty cycle (ratio of elements width to pitch) in a range of 0.40 to 0.60. However, as discussed, in connection with the example embodiments the figure of merit for the polarization recycling is given by either equation 1, equation 2, or equation 3 and not by the sum of Rs and Tp. Applicants have discovered that the figure of merit function of the example embodiment actually provides a duty cycle of the wire grid polarizer for polarization recycle that is quite different from the duty cycle of known wire grid polarizers having a duty cycle in the range of 0.40 to 0.60. To wit, and as will become clearer from the following description, the duty cycle of wire grid polarizers of the example embodiments is in the range of approximately 0.18 to approximately 0.25.
In the example embodiments, the improved total light throughput (T1 or T2) due to the improved transmittance Tp, at the expense of the reflectance Rs, is realized by providing a comparatively reduced width 204, or height 203, or both.
In order to further illustrate the example embodiments, the following graphical representations are described. It is noted that these graphical representations are illustrative of the benefits provided by the example embodiments, and are not intended to be limiting.
As can be readily appreciated, the transmittance Tp decreases with the duty cycle, and the reflectance Rs increases with the duty cycle. When the duty cycle is low, for example approaching approximately 0.0, the transmittance Tp is approximately 97%; however, the reflectance Rs is too low, only about 3%. At the opposite extreme, when the duty cycle is high, for example approaching approximately 1.00, the reflectance Rs is high, close to 92%, but the transmittance Tp is nearly 0%. Under either extreme case, the wire grid polarizer is not efficient in polarization recycling. Thus, a tradeoff must be made between the transmittance Tp and the reflectance Rs to maximize the total light throughput, such as described by equation 1 or equation 3, by varying the ratio of the width to pitch, and the ratio of the height to pitch.
As can be readily appreciated from a review of both curves 303 and 304, the function of merit is substantially optimal at a duty cycle of approximately 0.18 to approximately 0.25, with a peak value at approximately 0.22 for each system. Thus, the total light output of a display including a wire grid polarizer and a light recycling by diffusion reflection of light of one polarization state or by polarization transformation and reflection, is optimal if the wire grid polarizer has a duty cycle of approximately 0.18 to approximately 0.25. This is contrasted with the figures of merit for known devices having duty cycles of range of 0.40 to 0.60. As can be appreciated the figures of merit for these known wire grid polarizers, which are not useful for recycling, are significantly lower than the figures of merit (T1, T2) of the wire grid polarizer of the example embodiments.
In the preceding examples, R has been assumed to be 1 for simplified discussion. In the event that R is smaller than 1, Rs can be rescaled smaller to account for the impact of R. In the examples, T1 and T2 are beneficially improved, which clearly requires the improvement of the combination of the transmittance of the P-polarization (Tp) and the reflectance of the S-polarization (Rs). To this end, and as can be readily appreciated, the greater the transmittance of the P-polarization (Tp) and the greater the reflectance of the S-polarization (Rs), the higher the total throughput is.
The data shown in
As referenced previously, the angle of incidence to the polarizer may impact the light output of the wire grid polarizer.
Curve 601 is the transmittance versus θ for a wire grid polarizer having a duty cycle of 0.45, whereas the curve 602 is the transmittance for a wire grid polarizer of an example embodiment having a duty cycle of 0.20.
For an angle of incidence between approximately 0 degrees and 60 degrees, the transmittance is substantially the same for the wire grid polarizer of an example embodiment having a duty cycle of approximately 0.20.
Similarly, and as shown by curve 603 in
As can be readily appreciated from a review of
The average fill factor over the entire height H is given by:
Finally, it is noted that certain improvement in the light efficiency is realized through the improved light recycling afforded by the wire grid polarizers of the example embodiments. In keeping with the example embodiments, the display devices of the example embodiments of
In a further example embodiment depicted in
In another example embodiment, shown in
Wire grid polarizers are used in the back light unit for LCD to increase the optical efficiency in combination with other optical components. The overall optical efficiency can be optimized with the duty cycle specified above. However, other issues associated with the use of the wire grid polarizers in the back light unit need to be addressed as well.
Since the wire gird is placed in the vicinity of the light guide, it is often exposed to heat generated by the light source 104. A high thermal expansion coefficient of the wire gird polarizer leads to the distortion in the conductor arrangement and degrades image quality of the display. Humidity also can cause the same problem. One of the methods to prevent the distortion is to integrate the wire grid polarizer with other optical elements, for example, by lamination. The examples of the optical elements are collimating films, diffuser films, absorptive polarizer and light extraction film. Other mean of integration is also possible. For example, the substrate 201 can be optical elements. Such a direct integration of optical elements eliminates the number of film used in the back light unit and reduces the cost of LCD production and thickness of the unit. Common problems in handling the optical elements are dusts attracted to the surfaces of the optical elements due to the static charge. Since wire grid polarizer contain conductive wire, integration gives anti-static function. As it is well known in the art, a wire grid polarizer absorbs significant mount of light in ultra violet region. Therefore, the wire gird polarizer can prevent “yellowing” of the optical elements due to a cumulative UV radiation from the back light unit.
The integration can also be advantageous optically. By eliminating the air gap between wire grid polarizers and other optical components, the loss due to the Fresnel reflection is reduced. LCD television application requires less collimated light. In this case particularly, wire grid polarizer integrated with diffuser film is preferred. In the following, un-limiting examples of integration of wire grid polarizer and optical elements are described.
In one example embodiment, shown in
In another example embodiment, shown in
In another example embodiment, shown in
Wire grid polarizers having tilted conductors are more efficient for large angle light such as the case in wedge shaped light guide plate. The tilt of φ of the wire grid is optimized for various incident angle of the light.
As will be appreciated by one of ordinary skill in the art, by the example embodiments, light that would otherwise be lost may be reintroduced into the display system and thereby improve the brightness and contrast of the image formed. These and other benefits will become readily apparent to one of ordinary skill in the art from a review of the described example embodiments.
In accordance with illustrative embodiments, wire grids polarizers may be used in a display provide an improved optical efficiency (illuminance) compared to known structures that include specular reflectors over certain surfaces of the light guide. Further, the various methods, materials, components and parameters are included by way of example only and not in any limiting sense. Therefore, the embodiments described are illustrative and are useful in providing beneficial backlight assemblies. In view of this disclosure, those skilled in the art can implement the various example devices and methods to effect improved backlight efficiency, while remaining within the scope of the appended claims.